Method, apparatus for channel state information feedback and storage medium

ABSTRACT

A method and apparatus for channel state information feedback and a storage medium are provided according to the present disclosure. The method includes: performing by a terminal channel measurement according to a reference signal; selecting M subbands from K subbands of a CSI reporting band, and reporting to a base station the M subbands&#39; Relative Power Indicator (RPI) and Phase Indicator (PI) of a weighted coefficient associated with a precoding codebook index, where M and K are integers greater than or equal to 1, M&lt;K; each subband is a set of R resource blocks RB, where R is an integer greater than or equal to 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent document is a continuation of U.S. application Ser. No.16/455,722, filed on Jun. 27, 2019, entitled “METHOD, APPARATUS FORCHANNEL STATE INFORMATION FEEDBACK AND STORAGE MEDIUM” which is acontinuation of and claims the benefit of priority to InternationalPatent Application No. PCT/CN2018/090580, filed on Jun. 11, 2018, whichclaims the benefit of priority to Chinese Patent Application No.201710458924.X, filed on Jun. 16, 2017. The entire contents of thebefore-mentioned patent applications are incorporated by reference aspart of the disclosure of this application.

TECHNICAL FIELD

The present disclosure relates to, but is not limited to, the field ofchannel quality measurement.

BACKGROUND

In a wireless communication system, multiple antennas are used at thetransmitting end and the receiving end to achieve a higher rate. Theprinciple of Multiple-Input Multiple-Output (MIMO) is to establish amulti-layer transmission that matches the channel characteristics byusing some channel characteristics, thereby effectively improving systemperformance without increasing bandwidth and power. MIMO is a promisingtechnology that is widely used in current systems. For example, LongTerm Evolution (LTE) and Long Term Evolution-Advanced (LTE-A) as theenhancement version of LTE, include many multi-antenna transmissionmodes (Transmission Mode 2 to Transmission Mode 10).

In general, there are two Channel State Information (CSI) feedbackmechanisms, i.e., periodic feedback and aperiodic feedback. For example,in LTE system, the periodic feedback is performed on a Physical UplinkControl Channel (PUCCH); and the aperiodic feedback is performed on aPhysical Uplink Shared Channel (PUSCH). A terminal may report CSI in thefollowing two methods. The terminal may be configured by a base stationto measure and quantize channel information, and to periodically reporton the PUCCH the quantized CSI which includes Rank Indicator(RI)/Precoding Matrix Indicator (PMI)/Channel Quality Indication (CQI).Alternatively, the terminal may also be triggered aperiodically by thebase station to report CSI including RI/PMI/CQI when needed, mostly onthe PUSCH. Aperiodic feedback cures the problems of periodic feedbackhaving a poor real-time performance and the CSI quantization accuracybeing limited by the control channel overhead.

The basic principle of quantized channel information feedback based oncodebook is as follows.

Assuming that a limited feedback channel capacity is Bbps/Hz, and thenumber of codewords available is N=2B. Eigenvectors of the channelmatrix are quantized to form a codebook space

={F₁, F₂ . . . F_(N)}. The transmitting end and the receiving end mayboth store or generate in real time the codebook (the same at bothends). According to an obtained channel matrix H, the receiving endselects from

a codeword {circumflex over (F)} that best matches the channel based ona certain criteria, and reports a codeword sequence number i (i.e., PMI)to the transmitting end. The transmitting end finds the correspondingprecoding codeword {circumflex over (F)} according to the sequencenumber i, thereby obtaining channel information, where {circumflex over(F)} is eigenvector information of the channel.

The principle of codeword design in LTE is as follows:

LTE codebooks continue to evolve as the standard evolves. In Releases 8and 9, the 4-antenna codebook and the 2-antenna codebook aresingle-codeword codebooks, indicating one PMI, represented as i=1, . . ., N11, with N11 being the number of codewords. The 8-antenna codebook ofRelease 10 and the 4-antenna codebook of Release 12 are double-codewordcodebooks, written in the form of W=W1W2, where W1 is the codebook for along-term feedback, called a first codebook, and W2 is the codebook fora short-term feedback, called a second codebook. W2 is used forselecting one beam out of the M1 candidate beams in W1 codewords, andfor determining co-phasing for the selected beam in each polarizationdirection on the same data layer. Each codeword in W2 is quantized andreported by PMI2, represented as i2=1, . . . , M1, with M1 being thenumber of W2, which refers to LTE Release 10 for details.

Codewords before Release 12 are designed for a 1D antenna array,belonging to the category of 1D codewords. In Release 13, the dimensionof the codebook increases due to the use of more antennas. The topologyof the antennas is generally planar, i.e., 2D codewords are designed fora 2-dimensional antenna array. Therefore, each beam in the firstcodebook W1 has a two-dimensional form v_(m)⊗u_(n), where vm and un areDiscrete Fourier Transform (DFT) vectors of the first dimension antennaand the second dimension antenna respectively, and v_(m)⊗u_(n) denotesthe Kronecker product of vm and un, where m=1, 2, . . . , B1, and n=1,2, . . . , B2. The number of first-dimension ports (includingantennas/ports/transmission units/dipoles/array elements/any otherapparatus capable of sending signals) is N1; the number ofsecond-dimension ports is N2. DFT corresponding to the first-dimensionports is oversampled by a factor of O1; DFT corresponding to thesecond-dimension is oversampled by a factor of O2. The number of theaforementioned DFT vectors of the first dimension antenna or the seconddimension antenna is a multiple of the oversampling factor of the numberof ports, hence B1=N1*O1, B2=N2*O2, where O1 is the first-dimensionoversampling factor, and O2 is the second-dimension oversampling factor.The first-dimension codebook of the first codebook is indicated byPMI11, represented as i11=1, . . . , N11; and the codebook of thesecond-dimension of the first codebook is indicated by PMI12,represented as i12=1, . . . , N12. For each index value of the PMIs11and PMIs12, there are M1 of W2 codewords, each W2 codeword for selectingone 2-dimensional beam v_(m)⊗u_(n) out of W1 and for co-phasing inrespective polarization directions. The corresponding codeword index isPMI2, represented as i2=1, . . . , M1.

Without loss of generality, when the first-dimension port number N11=1or the second-dimension port number N12=1, the codeword becomes a 1Dcodeword; and when the first-dimension port number N11>1 and thesecond-dimension port number N12>1, the codeword becomes a 2D codeword.If it is a 1D codeword and has a single-codeword structure, the codewordis represented by PMI or i; if it is a 1D codeword and has adouble-codeword structure with PMI1 and PMI2, the index is representedby i1/i2; and if it is a 2D codeword, the codeword is jointlyrepresented by PMI11, PMI12 and PMI2 or indexes i11, i12 and i2.

In current systems, precoding matrix feedback and beam configuration arebased on strongest path information in the channel; information of otherpaths of the channel is ignored, resulting in mismatch between thefeedback or configured information and the channel, thereby affectingsystem performance. In order to solve this problem, the 3rd GenerationPartnership Project (3GPP) in its discussion over 5th-Generation (5G)introduces a codebook based on a linear combination of multipathinformation to New Radio Access (NR), which can greatly improve feedbackaccuracy and improve system performance. In a method for CSI feedbackthat uses a linear combination based codebook, channel information isquantized into a linear combination of multiple beams, i.e., theprecoding codeword of each layer is a linear combination of multipleone-dimensional or two-dimensional DFT vectors, where each of theone-dimensional or two-dimensional DFT vectors may be reported by a PMIin the above method. Each PMI corresponds to a weighted coefficienthaving an amplitude and a phase, where the amplitude may be reported byRelative Power Indicator (RPI), and the phase that may be reported byPhase Indicator (PI).

The codebook based on a combination of multiple paths can better matchthe channel, but has a significantly high overhead because thecombination requires amplitude coefficients and phase coefficients ofeach path be reported or configured. Moreover, a higher performance canbe achieved by RPI and PI feedback on subbands; but RPI and PI feedback,if on each subband, would bring in a greater feedback overhead.

SUMMARY

A method and apparatus for CSI feedback and a storage medium areprovided according to embodiments of the present disclosure.

The embodiments of the present disclosure include the technical detailsas follows.

A method for CSI feedback, applicable to a terminal, including:

performing channel measurement according to a reference signal; and

selecting M subbands from K subbands of a CSI reporting band, andreporting to a base station the M subbands' Relative Power Indicator(RPI) and/or Phase Indicator (PI) of a weighted coefficient associatedwith a precoding codebook index,

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks (RBs), where R is an integer greater thanor equal to 1.

The embodiments above may further include: determining the value of Maccording to a configuration signaling from the base station, or apre-defined rule.

In the embodiments above, the selecting M subbands from K subbands of aCSI reporting band, and reporting the RPIs and/or the PIs of M subbandsmay include at least one of:

selecting, according to an instruction signaling from the base station,from the K subbands, M subbands for PI and/or RPI reporting;

selecting, according to a pre-defined rule, from K subbands, M subbandsfor PI and/or RPI reporting; and

selecting, according to a result of channel measurement, from Ksubbands, M subbands for PI and/or RPI reporting.

In the embodiments above, the instruction signaling may include at leastone of:

a physical layer signaling, a Radio Resource Control (RRC) signaling, aMedium Access Control (MAC) signaling, Downlink Control Information(DCI), and a bitmap.

In the embodiments above, the selecting, according to a pre-definedrule, from K subbands M subbands, and reporting the PIs and/or RPIs ofthe M subbands include selecting according to a pre-defined rule, thepre-defined rule including at least one of:

selecting from the K subbands M subbands with the lowest frequencies;

selecting from the K subbands M subbands with the highest frequencies;

selecting from the K subbands M subbands containing subbands with thehighest frequency and the lowest frequency; and

selecting M subbands which are in a comb-shaped distribution in the Ksubbands.

In the embodiments above, the selecting according to a result of channelmeasurement from K subbands M subbands, and reporting the PIs and/orRPIs of the M subbands may include:

calculating CQIs of the K subbands, and selecting M subbands of the Ksubbands having the best CQIs for PI and/or RPI reporting.

The embodiments above may further include:

reporting to the base station numbering information of the M subbands ofthe K subbands having the best CQIs.

In the embodiments above, the calculating CQIs of the K subbands mayinclude:

calculating CQIs, according to Precoding Matrix Indicators (PMIs), RPIsand PIs obtained from the channel measurement.

In the embodiments above, the selecting from K subbands of a CSIreporting band M subbands for feedback of PIs and/or RPIs may include:

dividing the K subbands into N subband sets; and

determining one or more subbands from each of the subband sets, to formM subbands as the subbands for PI and/or RPI reporting.

In the embodiments above, the dividing the K subbands of the CSIreporting band into N subband sets may include dividing according to atleast one of:

a signaling from the base station, an pre-defined rule, and a result ofchannel measurement.

In the embodiments above, the dividing the K subbands into N subbandsets and determining one or more subbands from each of the subband setsmay include:

dividing K subbands into N subband sets, where the n-th subband setincludes Kn subbands; and

determining Mn subbands from the n-th subband set, as subbands for PI orRPI reporting, where n is an integer greater than or equal to 1, andn≤N.

The embodiments above may further include determining the value of Mnaccording to a configuration signaling from a base station or apre-defined rule.

In the embodiments above, the determining one or more subbands from eachof the subband sets may include at least one of:

selecting, according to an instruction signaling from a base station, acorresponding number of subbands from each of the subband sets for PIand/or RPI reporting;

selecting, according to a pre-defined rule, a corresponding number ofsubbands from each of the sub-band sets for PI and/or RPI reporting; and

selecting, according to a result of the channel measurement, acorresponding number of subbands from each of the sub-band sets for PIand/or RPI reporting.

In the embodiments above, the selecting according to a result of thechannel measurement a corresponding number of subbands from each of thesub-band sets for PI or RPI reporting may include:

calculating CQIs of the K subbands; and

selecting, from the Kn subbands of the n-th subband set, Mn subbandshaving the highest CQIs for reporting at least one of a correspondingPI, a corresponding RPI and information indicating the Mn subbands.

In the embodiments above, the selecting, according to a pre-definedrule, from the Kn subbands of the n-th subband set Mn subbands forreporting a corresponding PI and/or RPI may include at least one of:

selecting Mn subbands with the lowest frequencies from the Kn subbands;

selecting Mn subbands with the highest frequencies from the Kn subbands;

selecting from the Kn subbands Mn subbands containing subbands with thehighest frequency and the lowest frequency; and

selecting Mn subbands which are in a comb-shaped distribution in the Knsubbands.

The embodiments above may further include dividing the K subbands into Nsubband sets according to an pre-defined rule including at least:

dividing the K subbands into N subband sets evenly.

In the embodiments above, the dividing the K subbands into N subbandsets according to a result of channel measurement may include at leastone of:

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index, whereeach information differs from one another by within DR or DP, DR or DPbeing a predetermined value or determined by a signaling from the basestation; and

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index, wherethe information varies between the subbands in the subband set within ERor EP, ER or EP being a predetermined value or determined by a signalingfrom the base station.

The embodiments above may further include:

reporting to the base station a value of at least one of: N, Kn, and Mn.

A method for Channel State Information (CSI) feedback, applicable to abase station, is provided according to an embodiment of the presentdisclosure, which includes:

receiving, from a terminal, M subbands' Relative Power Indicator (RPI)and/or Phase Indicator (PI) of a weighted coefficient associated with aprecoding codebook index, and where the M subbands are selected by theterminal from K subbands of a CSI reporting band,

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks (RBs), and R is an integer greater than orequal to 1.

The embodiments above may further include:

notifying the terminal of a value of M, by a configuration signaling.

The embodiments above may further include:

notifying the terminal of order information of a comb-shapeddistribution of M subbands in the K subbands, by a configurationsignaling.

The embodiments above may further include:

notifying the terminal to select from the K subbands M subbands for PIand/or RPI reporting, by an instruction signaling.

The instruction signaling may include at least one of:

a physical layer signaling, a Radio Resource Control (RRC) signaling, aMedium Access Control (MAC) signaling, Downlink Control Information(DCI), and a bitmap.

The embodiments above may further include:

receiving from the terminal numbering information of the M subbands ofthe K subbands.

The embodiments above may further include:

notifying the terminal to divide the K subbands of the reportingbandwidth into N subband sets.

The embodiments above may further include:

notifying the terminal to determine one or more subbands from each ofthe subband sets, to form M subbands as the subbands for PI and/or RPIreporting.

The embodiments above may further include:

notifying the terminal to divide the K subbands of the reportingbandwidth into N subband sets, where the n-th subband set includes Knsubbands.

The embodiments above may further include:

notifying the terminal to determine Mn subbands from the n-th subbandset, as subbands for PI or RPI reporting, where n is an integer greaterthan or equal to 1, and n≤N.

The embodiments above may further include:

receiving from the terminal at least one of: the number of subband sets,the number of subbands in each subband set, and the number of subbandsfor PI and/or RPI reporting in each subband set.

An apparatus for Channel State Information (CSI) feedback, provided at aterminal, is provided according to an embodiment of the presentdisclosure, which includes:

a measuring module, configured to perform channel measurement accordingto a reference signal; and

a feedback module, configured to select, from K subbands of a CSIreporting band, M subbands, and report to the base station the Msubbands' PI and/or RPI of a weighted coefficient associated with aprecoding codebook index,

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks (RBs), where R is an integer greater thanor equal to 1.

In the embodiments above, the feedback module may further be configuredto determine the value of M according to a configuration signaling fromthe base station, or a pre-defined rule.

In the embodiments above, the feedback module may further be configuredto select from the K subbands of the CSI reporting band M subbands forfeedback of PI and/or RPI reporting, by at least one of:

selecting, according to an instruction signaling from the base station,from the K subbands, M subbands for PI and/or RPI reporting;

selecting, according to a pre-defined rule, from the K subbands, Msubbands for PI and/or RPI reporting; and

selecting, according to a result of channel measurement, from the Ksubbands, M subbands for PI and/or RPI reporting.

In the embodiments above, the feedback module may further be configuredto select, according to a pre-defined rule, from the K subbands, Msubbands for PI and/or RPI reporting, where the pre-defined ruleincludes at least one of:

selecting from the K subbands M subbands with the lowest frequencies;

selecting from the K subbands M subbands with the highest frequencies;

selecting from the K subbands M subbands containing subbands with thehighest frequency and the lowest frequency; and

selecting M subbands which are in a comb-shaped distribution in the Ksubbands.

In the embodiments above, the feedback module may further be configuredto select according to a result of channel measurement from the Ksubbands M subbands for PI and/or RPI reporting, by:

calculating CQIs of the K subbands, and selecting M subbands of the Ksubbands having the best CQIs for PI and/or RPI reporting.

In the embodiments above, the feedback module may further be configuredto report to the base station numbering information of the M subbands ofthe K subbands having the best CQIs.

In the embodiments above, the feedback module may further be configuredto calculate the CQIs of the K subbands, by:

calculating the CQIs, according to Precoding Matrix Indicators (PMIs),RPIs and PIs obtained from the channel measurement.

In the embodiments above, the feedback module may further be configuredto select from the K subbands of the CSI reporting band M subbands forfeedback of PI and/or RPI, by:

dividing the K subbands into N subband sets; and

determining one or more subbands from each of the subband sets, to formM subbands as the subbands for PI and/or RPI reporting.

In the embodiments above, the feedback module may further be configuredto divide the K subbands of the CSI reporting band into N subband setsaccording to at least one of:

a signaling from the base station, an pre-defined rule, and a result ofchannel measurement.

In the embodiments above, the dividing K subbands into N subband setsand determining one or more subbands from each of the subband sets mayinclude:

dividing K subbands into N subband sets, where the n-th subband setincludes Kn subbands;

determining Mn subbands from the n-th subband set, as subbands for PI orRPI reporting, where n is an integer greater than or equal to 1, andn≤N.

In the embodiments above, the feedback module may further be configuredto determining the value of Mn according to a configuration signalingfrom a base station, or a pre-defined rule.

In the embodiments above, the feedback module may further be configuredto determine one or more subbands from each of the subband sets, by atleast one of:

selecting, according to an instruction signaling from a base station, acorresponding number of subbands from each of the subband sets for PIand/or RPI reporting;

selecting, according to a pre-defined rule, a corresponding number ofsubbands from each of the sub-band sets for PI and/or RPI reporting; and

selecting, according to a result of the channel measurement, acorresponding number of subbands from each of the sub-band sets for PIand/or RPI reporting.

In the embodiments above, the feedback module may further be configuredto select according to the result of the channel measurement acorresponding number of subbands from each of the sub-band sets for PIand/or RPI reporting, by:

calculating CQIs of the K subbands; and

selecting, from the Kn subbands of the n-th subband set, Mn subbandshaving the highest CQIs for reporting at least one of a correspondingPI, a corresponding RPI, and information indicating the Mn subbands.

In the embodiments above, the feedback module may further be configuredto select according to a pre-defined rule Mn subbands from the Knsubbands of the n-th subband set for reporting the corresponding PIand/or RPI, by at least one of:

selecting Mn subbands with the lowest frequencies from the Kn subbands;

selecting Mn subbands with the highest frequencies from the Kn subbands;

selecting from the Kn subbands Mn subbands containing subbands with thehighest frequency and the lowest frequency; and

selecting Mn subbands which are in a comb-shaped distribution in the Knsubbands.

In the embodiments above, the feedback module may further be configuredto divide the K subbands into the N subband sets according to anpre-defined rule by at least:

dividing the K subbands into N subband sets evenly.

In the embodiments above, the feedback module may further be configuredto divide the K subbands into the N subband sets according to the resultof channel measurement, by at least one of:

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index, whereeach information differs from one another by within DR or DP, DR or DPbeing a predetermined value or determined by a signaling from the basestation; and

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index, wherethe information varies between the subbands in the subband set within ERor EP, ER or EP being a predetermined value or determined by a signalingfrom the base station.

In the embodiments above, the feedback module may further be configuredto report to the base station a value of at least one of: N, Kn, and Mn.

An apparatus for Channel State Information (CSI) feedback, provided at abase station, is also provided according to an embodiment of the presentdisclosure, which includes:

a communication module, configured to receive, from a terminal, Msubbands' Phase Indicator (PI) and/or Relative Power Indicator (RPI) ofa weighted coefficient associated with a precoding codebook index of Msubbands, the M subbands being selected by the terminal from K subbandsof a CSI reporting band,

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of

R resource blocks (RBs), where R is an integer greater than or equal to1.

In the embodiments above, the communication module may further beconfigured to notify the terminal of a value of M, by a configurationsignaling.

In the embodiments above, the communication module may further beconfigured to notify the terminal of order information of a comb-shapeddistribution of M subbands in the K subbands, by a configurationsignaling.

In the embodiments above, the communication module may further beconfigured to notify the terminal to select from the K subbands Msubbands for PI and/or RPI reporting, by an instruction signaling.

In the embodiments above, the communication module may further beconfigured to receive from the terminal numbering information of the Msubbands of the K subbands.

In the embodiments above, the communication module may further beconfigured to notify the terminal to divide the K subbands of the CSIreporting band into N subband sets.

In the embodiments above, the communication module may further beconfigured to notify the terminal to determine one or more subbands fromeach of the subband sets, to form M subbands as the subbands for PIand/or RPI reporting.

In the embodiments above, the communication module may further beconfigured to notify the terminal to divide the K subbands of thereporting bandwidth into N subband sets, where the n-th subband setincludes Kn subbands.

In the embodiments above, the communication module may further beconfigured to notify the terminal to determine Mn subbands from the n-thsubband set, as subbands for PI or RPI reporting, where n is an integergreater than or equal to 1, and n≤N.

In the embodiments above, the communication module may further beconfigured to receive from the terminal at least one of: the number ofsubband sets, the number of subbands in each subband set, and the numberof subbands for PI and/or RPI reporting in each subband set.

A storage medium is also provided according to an embodiment of thepresent disclosure, which have a computer program stored therein, thecomputer program executable by a processor for performing the steps ofany one of terminal-side methods, or performing the steps of any one ofbase station-side methods.

In comparison with the existing technology, the embodiments of thepresent disclosure show the following advantageous effects:

In the technical embodiments of the present disclosure, instead ofreporting the RPI and PI of every subband, only a portion of thesubbands have their RPIs and PIs reported. Therefore, feedback overheadis reduced. Additionally, CSI feedback performance can be maintainedbecause amplitude and phase information indicated by PI and RPI show acertain correlation between different subbands, and the base station caninterpolate information of the rest of the subbands from the M subbands,thereby solving the problem of significant CSI feedback overhead withlinear combination codebook based feedback.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a linear combination codebook basedfeedback according to an embodiment of the present disclosure;

FIG. 2 is a flowchart of a method for CSI feedback according to anembodiment of the present disclosure;

FIG. 3 is a flowchart of another method for CSI feedback according to anembodiment of the present disclosure;

FIG. 4 is a schematic structural diagram of an apparatus for CSIfeedback according to an embodiment of the present disclosure;

FIG. 5 is a schematic structural diagram of an apparatus for CSIfeedback according to an embodiment of the present disclosure;

FIG. 6 is a schematic diagram of subband feedback according toEmbodiment 1 of the present disclosure;

FIG. 7 is a schematic diagram of subband feedback according toEmbodiment 2 of the present disclosure; and

FIG. 8 is a schematic diagram of the comb-shaped distribution accordingto Embodiment 1 of the present disclosure.

DETAILED DESCRIPTION

For a better understanding of the objects, technical details andadvantageous effects of the present disclosure, embodiments of thepresent disclosure will be described hereinafter in detail withreference to the accompanying drawings. Features in the embodiments andthe embodiments themselves may be combined in any manner that does notcause conflict.

As shown in FIG. 1 , in a method of CSI feedback using a linearcombination based codebook, the channel information measured by theterminal according to a reference signal is quantized into a linearcombination of multiple beams, i.e., the precoding codeword of eachlayer is a linear combination of multiple one-dimensional ortwo-dimensional DFT vectors. In FIG. 1 , the selected beam vectors c₀,c₁, and c₂ may be reported by a conventional PMI, where the coefficientsused in weighing the PMIs include amplitude information λ₁ and λ₂ andphase information α₁ and α₂, with the amplitude information reported byRPI and the phase information reported by PI. In comparison with aconventional single-path codebook PMI feedback, the feedback overheadhere is greatly increased due to the need to report the RPI and PIinformation corresponding to each beam.

Generally, beam information in a wireless channel is less frequencyselective. Therefore, in linear combination based codebook feedback,beam PMI information indicating a base vector may be reported to thebase station using wideband feedback. The phase of the weightedcoefficient is susceptible to such factors as delay and the randomphase, hence is more frequency selective; therefore, subband feedback isrequired for PI. For the amplitude, i.e. RPI, of the weightedcoefficient, wideband feedback can ensure a good performance; subbandfeedback can improve the performance, but would bring in a greatoverhead. Therefore, wideband feedback or subband feedback can beconfigured according to needs. In summary, a high-performance linearcombination based codebook feedback requires subband feedback of RPI andPI for each beam.

In order to realize subband feedback of RPI and PI, a simpler method isto report the RPI and PI corresponding to each subband, which brings ina great amount of performance overhead. The phase of the weightedcoefficient indicated by PI may be affected by mainly two factors: (a)the random initial phase; and (b) delays that cause a phase shift in thefrequency domain. For the random initial phase, phase change in eachpolarization direction can be seen as a fixed value on each subband. Fordelays, the phase shift in the frequency domain can be seen as varieslinearly with the frequency. Therefore, change of the phase indicated byPI between different frequency bands can be modeled using a linear,piecewise linear or some other function. The amplitude informationindicated by RPI may only be affected by some small random factors indifferent frequency bands; overall it is stable. In summary, theamplitude and phase indicated by the PI and RPI corresponding to thesame beam show a certain correlation between different subbands.

According to an embodiment of the present disclosure, the correlationbetween different subbands of the amplitude and phase indicated by thePI and RPI is used to reduce the overhead of subband feedback. A simplemethod is to provide the RPI and PI on only a portion of the subbands,instead of reporting the RPI and PI on each of the subbands, therebyproviding a subband feedback method with reduced overhead.

As shown in FIG. 2 , a method for CSI feedback, applicable to aterminal, is provided according to an embodiment of the presentdisclosure. The method includes:

S101. performing channel measurement according to a reference signal;

S102. selecting, from K subbands of a CSI reporting band, M subbands forfeedback of PI and/or RPI of a weighted coefficient associated with aprecoding codebook index;

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks (RBs), where R is an integer greater thanor equal to 1.

In the embodiment of the present disclosure, the reporting bandwidth forPI and/or RPI includes a total of K subbands; the terminal reports RPIand/or PI corresponding to M subbands of the K subbands, where M is aninteger greater than 0 and less than K, therefore feedback overhead isreduced. In addition, because amplitude and phase information indicatedby PI and RPI show a certain correlation between different subbands, andthe base station can interpolate the information of the rest of thesubbands from the M subbands, the CSI feedback performance can bemaintained.

The value of M is determined, according to at least one of: aconfiguration signaling from a base station, or a pre-defined rule.

In step S102, the M subbands for feedback of PI and/or RPI are selectedfrom K subbands of the CSI reporting band, by at least one of:

Method 1: selecting, according to a signaling message from the basestation, from the K subbands of the CSI reporting band, M subbands forPI and/or RPI reporting;

Method 2: selecting, according to a pre-defined rule, from the Ksubbands of the CSI reporting band, M subbands for PI and/or RPIreporting;

Method 3: selecting, according to a result of channel measurement, fromthe K subbands of the CSI reporting band, M subbands for PI and/or RPIreporting; and

Method 4: dividing the K subbands of the CSI reporting band into Nsubband sets; and determining one or more subbands from each of thesubband sets, to form M subbands as the subbands for PI and/or RPIreporting.

In Method 1, the signaling message may include at least one of: aphysical layer signaling, an RRC signaling, an MAC signaling, DCI, and abitmap.

Method 2 includes: selecting, according to a pre-defined rule, from Ksubbands, M subbands for PI and/or RPI reporting, where the pre-definedrule includes at least one of:

selecting from the K subbands M subbands with the lowest frequencies;

selecting from the K subbands M subbands with the highest frequencies;

selecting from the K subbands M subbands containing subbands with thehighest and frequency and the lowest frequency; and

selecting M subbands which are in a comb-shaped distribution in the Ksubbands.

In Method 3, the selecting according to the result of channelmeasurement from K subbands M subbands for PI and/or RPI reportingincludes:

calculating CQIs of the K subbands, and selecting M subbands of the Ksubbands having the best CQIs for PI and/or RPI reporting.

According to an embodiment, in Method 3, the terminal may further reportto the base station numbering information of the M subbands of the Ksubbands having the best CQIs. The calculation of CQIs of the K subbandsmay include: calculating CQIs, according to PMI, RPI and PI obtained bychannel measurement.

In Method 4, the dividing K subbands of a CSI reporting band into Nsubband sets includes dividing according to at least one of:

a signaling message from the base station, an pre-defined rule, and aresult of channel measurement.

In Method 4, the dividing K subbands into N subband sets and determiningone or more subbands from each of the subband sets include:

dividing K subbands of the reporting bandwidth into N subband sets,where the n-th subband set includes Kn subbands; and determining Mnsubbands from the n-th subband set, as subbands for PI or RPI reporting,n is an integer greater than or equal to 1, and n≤N. The value of Mn isdetermined according to at least one of: a configuration signaling froma base station, and a pre-defined rule.

According to an embodiment, the terminal reports to the base station avalue of at least one of: N, Kn, and Mn.

One or more subbands are determined from each of the subband sets by atleast one of:

selecting, according to a signaling message from a base station, acorresponding number of subbands from each of the subband sets for PIand/or RPI reporting;

selecting, according to a pre-defined rule, a corresponding number ofsubbands from each of the sub-band sets for PI and/or RPI reporting; and

selecting, according to a result of the channel measurement, acorresponding number of subbands from each of the sub-band sets for PIand/or RPI reporting.

Specifically, the selecting according to a result of the channelmeasurement a corresponding number of subbands from each of the sub-bandsets for PI and/or RPI reporting may include:

calculating CQIs of the K subbands; and selecting, from the Kn subbandsof the n-th subband set, Mn subbands having the highest CQIs forreporting at least one of a corresponding PI, a corresponding RPI andinformation indicating the Mn subbands.

Specifically, the selecting according to a predetermined method Mnsubbands from the Kn subbands of the n-th subband set for reporting acorresponding PI and/or RPI may include at least one of:

selecting Mn subbands with the lowest frequencies from the Kn subbands;

selecting Mn subbands with the highest frequencies from the Kn subbands;

selecting, from the Kn subbands, Mn subbands containing subbands withthe highest frequency and a lowest frequency; and

selecting Mn subbands which are in a comb-shaped distribution in the Knsubbands.

The dividing K subbands into N subband sets according to a predeterminedmethod may include at least:

dividing the K subbands into N subband sets evenly.

The dividing K subbands into N subband sets according to a result ofchannel measurement may include at least one of:

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index whereeach information differs from one another by within DR or DP, DR or DPbeing a predetermined value or determined by a signaling message from abase station;

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index wherethe information varies between the subbands in the subband set within ERor EP, ER or EP being a predetermined value or determined by a signalingfrom a base station.

As shown in FIG. 3 , a method for CSI feedback, applicable to a basestation, is also provided according to an embodiment of the presentdisclosure. The method includes:

S201. receiving, from a terminal, M subbands' PI and/or RPI of aweighted coefficient associated with a precoding codebook index, the Msubbands being selected by the terminal from K subbands of a CSIreporting band;

S202. determining CSI of the K subbands, according to a correlation ofan amplitude and the phase among subbands indicated by the PI and/or RPIof the M subbands;

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks RB, where R is an integer greater than orequal to 1.

The base station may notify the terminal of the value of M, by aconfiguration signaling.

The base station may notify the terminal of order information of acomb-shaped distribution of M subbands in the K subbands, by aconfiguration signaling.

The base station may notify the terminal to select from the K subbands Msubbands for PI and/or RPI reporting, by a signaling message.

Specifically, the signaling message may include at least one of: aphysical layer signaling, an RRC signaling, a MAC signaling, DCI, and abitmap.

The base station may further receive from the terminal numberinginformation of the M subbands of the K subbands.

According to an embodiment, the notifying the terminal to select fromthe K subbands M subbands for PI and/or RPI reporting by a signalingmessage includes:

notifying the terminal to divide K subbands of the reporting bandwidthinto N subband sets; and determining one or more subbands from each ofthe subband sets, to form M subbands as the subbands for PI and/or RPIreporting.

This method may further include: notifying the terminal to divide the Ksubbands of a CSI reporting band into N subband sets, where the n-thsubband set includes Kn subbands; and determining Mn subbands from then-th subband set, as subbands for PI or RPI reporting, n is an integergreater than or equal to 1, and n≤N.

The base station may receive from the terminal at least one of: thenumber of subband sets, the number of subbands in each subband set, andthe number of subbands for PI or RPI reporting in each subband set.

To realize the methods of the embodiments of the present disclosure, asshown in FIG. 4 , an apparatus for CSI feedback, provided at a terminal,is provided according to according to an embodiment of the presentdisclosure. The apparatus includes:

a measuring module, configured to perform channel measurement accordingto a reference signal;

a feedback module, configured to select, from K subbands of a CSIreporting band, M subbands, and report to a base station the M subbands'PI and/or RPI of a weighted coefficient associated with a precodingcodebook index;

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks RB, where R is an integer greater than orequal to 1.

The feedback module may further be configured to determine the value ofM according to a configuration signaling from a base station, or apre-defined rule.

The feedback module may select from the K subbands of the CSI reportingband M subbands for feedback of PI and/or RPI, by at least one of:

selecting, according to a signaling message from the base station, fromthe K subbands of the CSI reporting band, M subbands for PI and/or RPIreporting;

selecting, according to a pre-defined rule, from the K subbands of theCSI reporting band, M subbands for PI and/or RPI reporting;

selecting, according to a result of channel measurement, from the Ksubbands of the CSI reporting band, M subbands for PI and/or RPIreporting.

The feedback module may select, according to a pre-defined rule, from Ksubbands, M subbands for PI and/or RPI reporting, where the pre-definedrule includes at least one of:

selecting M subbands with the lowest frequencies from the K subbands;

selecting M subbands with the highest frequencies from the K subbands;

selecting from the K subbands M subbands containing subbands with thehighest frequency and the lowest frequency; and

selecting M subbands which are in a comb-shaped distribution in the Ksubbands.

The feedback module may select according to a result of channelmeasurement from K subbands M subbands for PI and/or RPI reporting, by:

calculating CQIs of the K subbands, and selecting M subbands of the Ksubbands having the best CQIs for PI and/or RPI reporting.

The feedback module may further be configured to report to the basestation numbering information of the M subbands of the K subbands havingthe best CQIs.

The feedback module may calculate the CQIs of the K subbands, by:

calculating the CQIs, according to Precoding Matrix Indicators (PMIs),Relative Power Indicators (RPIs) and Phase Indicators (PIs) obtainedfrom the channel measurement.

The feedback module may divide the K subbands of the CSI reporting bandinto N subband sets according to at least one of:

a signaling from the base station, an pre-defined rule, and a result ofchannel measurement.

The dividing K subbands into N subband sets and determining one or moresubbands from each of the subband sets include:

dividing K subbands of the reporting bandwidth into N subband sets,where the n-th subband set includes Kn subbands; and determining Mnsubbands from the n-th subband set, as subbands for PI or RPI reporting,n is an integer greater than or equal to 1, and n≤N.

Determining the value of Mn includes determining according to aconfiguration signaling from a base station, or a pre-defined rule.

The feedback module may determine one or more subbands from each of thesubband sets, by at least one of:

selecting, according to a signaling message from a base station, acorresponding number of subbands from each of the subband sets for PI orRPI reporting;

selecting, according to a pre-defined rule, a corresponding number ofsubbands from each of the sub-band sets for PI or RPI reporting; and

selecting, according to a result of the channel measurement, acorresponding number of subbands from each of the sub-band sets for PIor RPI reporting.

The feedback module may select according to the result of the channelmeasurement the corresponding number of subbands from each of thesub-band sets for PI and/or RPI reporting, by:

calculating CQIs of the K subbands, and selecting, from the Kn subbandsof the n-th subband set, Mn subbands having the highest CQIs forreporting at least one of a corresponding PI, a corresponding RPI, andinformation indicating the Mn subbands.

The feedback module may select according to a predetermined method Mnsubbands from the Kn subbands of the n-th subband set for reporting thecorresponding PI and/or RPI, by at least one of:

selecting Mn subbands with the lowest frequencies from the Kn subbands;

selecting Mn subbands with the highest frequencies from the Kn subbands;

selecting, from the Kn subbands, Mn subbands containing subbands withthe highest frequency and the lowest frequency; and

selecting Mn subbands which are in a comb-shaped distribution in the Knsubbands.

The feedback module may divide the K subbands into N subband setsaccording to an pre-defined rule by at least:

dividing the K subbands into N subband sets evenly.

The feedback module may divide K subbands into N subband sets accordingto a result of channel measurement, by at least one of:

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index whereeach information differs from one another by within DR or DP, DR or DPbeing a predetermined value or determined by a signaling from the basestation; and

forming subband sets, each of which includes subbands having informationindicated by RPI or PI associated with a precoding codebook index wherethe information varies between the subbands in the subband set within ERor EP, ER or EP being a predetermined value or determined by a signalingfrom the base station.

The feedback module may report to the base station a value of at leastone of: N, Kn, and Mn.

In practice, the measurement module and the feedback module may beimplemented with a processor in conjunction with a communicationinterface in an apparatus for CSI feedback.

To realize the methods of the embodiments of the present disclosure, asshown in FIG. 5 , an apparatus for CSI feedback, provided at a basestation, is provided according to an embodiment of the presentdisclosure. The apparatus includes:

a communication module, configured to receive, from a terminal, Msubbands' PI and/or RPI of a weighted coefficient associated with aprecoding codebook index, the M subbands being selected by the terminalfrom K subbands of a CSI reporting band;

a determination module, configured to determine CSI of the K subbands,according to a correlation of the amplitude and the phase among subbandsindicated by the PI and/or RPI of the M subbands;

where M and K are integers greater than or equal to 1, M<K; each subbandis a set of R resource blocks (RBs), where R is an integer greater thanor equal to 1.

The communication module may further be configured to notify theterminal of the value of M, by a configuration signaling.

The communication module may further be configured to notify theterminal of order information of a comb-shaped distribution of Msubbands in the K subbands, by a configuration signaling.

The communication module may further be configured to notify theterminal to select from the K subbands M subbands for PI and/or RPIreporting, by a signaling message.

The communication module may further be configured to receive from theterminal numbering information of the M subbands of the K subbands.

The communication module may further be configured to notify theterminal to divide

K subbands of the CSI reporting band into N subband sets.

The communication module may further be configured to notify theterminal to determine one or more subbands from each of the subbandsets, to form M subbands as the subbands for PI and/or RPI reporting.

The communication module may further be configured to notify theterminal to divide the K subbands of a CSI reporting band into N subbandsets, where the n-th subband set includes

Kn subbands.

The communication module may further be configured to notify theterminal to determine Mn subbands from the n-th subband set, as subbandsfor PI or RPI reporting, n is an integer greater than or equal to 1, andn≤N.

The communication module may further be configured to receive from theterminal at least one of: the number of subband sets, the number ofsubbands in each subband set, and the number of subbands for PI or RPIreporting in each subband set.

In practice, the communication module may be implemented with aprocessor in conjunction with a communication interface in an apparatusfor CSI feedback; the determination module may be implemented with aprocessor in an apparatus for CSI feedback.

It should be noted that, the separation of program modules inside theCSI feedback apparatus is described above according to the embodimentsby way of example; in practice, other forms of separation of programmodules may be implemented. In other words, the internal structure ofthe apparatus may be divided into a different set of program modules, tocomplete all or part of the processing described above. In addition, theCSI feedback apparatus provided in the embodiments above relates to thesame invention as the CSI feedback method implementation below; hencethe details of apparatus implementation will be omitted.

Embodiment 1

With reference to FIG. 6 , this embodiment shows the selecting from Ksubbands of a CSI reporting band M subbands may include:

determining, by a terminal, the value of M and M subbands from the Ksubbands in various methods.

Method 1: Signaling notification from the base station. A base stationmay notify by a higher-layer or physical-layer signal, the terminal toreport PRI and/or PI on M subbands of the K subbands. For example, thebase station may notify by a bitmap the terminal of how to select Msubbands from the K subbands. The signaling may be a higher-layersignaling such as an RRC or MAC signaling, to achieve a semi-static,semi-persistent configuration; or, a PHY signaling such as DCI, toachieve a dynamic configuration.

Method 2: Pre-defined rule. The terminal may select, according to apre-defined rule, from K subbands, M subbands that are relatively fixed,for example:

1. the M subbands may be subbands of the K subbands having the lowestfrequencies;

2. the M subbands may be subbands of the K subbands having the highestfrequencies;

3. the M subbands may contain subbands of the K subbands having thehighest and lowest frequencies;

4. the M subbands may have a comb-shaped distribution in the K subbands(shown in FIG. 8 ), in which case the base station may determine thevalue of M by notifying the order of a comb-shaped distribution bysignaling.

Method 3: The terminal determines on its own based on a result ofchannel measurement. The terminal performs channel measurement accordingto a reference signal, calculates RPI and/or PI of each subband,calculates CQI of each subband based on the RPI, RI, PI, PMI, etc., andreport the RPIs and/or PIs of the M subbands having the best CQIs andreport the corresponding numbering of the M subbands.

The methods above can reduce the overhead on subband feedback of PRI andPI, while maintaining the CSI feedback performance.

Embodiment 2

This embodiment shows a particular example of channel informationquantization and feedback. According to the embodiment, K subbands of aCSI reporting band are divided into N subband sets, each subband setincluding Kn subbands. In RPI and PI feedback, the terminal selects Mnsubbands from the Kn subbands of the n-th subband set, for reporting acorresponding PI or RPI, shown in FIG. 7 .

The terminal may determine how to select Mn subbands from the Knsubbands of the n-th subband set for reporting a corresponding PI or RPIin various methods.

Method 1: Signaling notification from the base station. A base stationmay notify by a higher-layer or physical-layer signal, the terminal toreport PRI and/or PI on Mn subbands of the Kn subbands. For example, thebase station may notify by a bitmap the terminal of how to select Mnsubbands from the Kn subbands. The signaling may be a higher-layersignaling such as an RRC or MAC signaling, to achieve a semi-static,semi-persistent configuration; or, an PHY signaling such as DCI, toachieve a dynamic configuration.

Method 2: Pre-defined rule. The terminal may select, according to apre-defined rule, from Kn subbands, Mn subbands that are relativelyfixed, for example:

1. the Mn subbands may be subbands of the Kn subbands having the lowestfrequencies;

2. the Mn subbands may be subbands of the Kn subbands having the highestfrequencies;

3. the Mn subbands may contain subbands of the Kn subbands having thehighest and lowest frequencies;

4. the Mn subbands may have a comb-shaped distribution in the Knsubbands, in which case the base station may determine the value of M bynotifying the order of a comb distribution by signaling.

Method 3: The terminal determines on its own based on a result ofchannel measurement. The terminal performs channel measurement accordingto a reference signal, calculates RPIs and/or PIs of the Kn subbands inthe n-th subband set, calculates CQIs of the Kn subbands based on theRPI, RI, PI, PMI, etc., and report the RPIs and/or PIs of the M subbandshaving the best CQIs and report the corresponding numbering of the Msubbands.

Another problem to be solved by the method is the division of subbandsets. The terminal may determine the division of subband sets in thefollowing three methods.

Method A: The terminal may determine the values of N, Kn and Mnaccording to an instruction signaling from the base station. The signalmay be a higher-layer signaling such as an RRC or MAC signaling, toachieve a semi-static, semi-persistent configuration; or, an PHYsignaling such as DCI, to achieve a dynamic configuration.

Method B: The terminal may determine the values of N, Kn and Mnaccording to a pre-defined rule including at least one of:

1. Mn equals to 1 or 2, i.e., 1 or 2 subbands are selected from eachsubband set for RPI and/or PI reporting;

2. Kn is a constant for n=1, . . . , N, Kn, i.e., the K subbands aredivided evenly;

3. Mn is a constant for n=1, . . . , N, Kn.

Method C: The terminal may determine the values of N, Kn and Mnaccording to a channel measurement. The terminal may calculate RPI andPI on each subband, and group subbands with RPIs and PIs in a certainrange together to form a subband set. Specifically, the terminal maycalculate RPIs and PIs on K subbands, and group subbands to form thesubband set. RPI or PI of subband in the subband set differs from eachother by within DR or DP. DR or DP are predetermined values ordetermined by a signaling from the base station. The terminal may reportthe values of N, Kn and Mn. A special case is when the K subbands aredivided evenly into N subband sets; in this case the terminal only needsto report the value of N.

Method D: The terminal may determine the values of N, Kn and Mnaccording to a channel measurement. The terminal may calculate RPIand/or PI on each subband, and group subbands with RPIs and PIs meetinga certain rule together, to form a subband set. The terminal may groupsubbands having information indicated by RPI and/or PI, together to forma subband set, where the information has similar variation.Specifically, if RPIs and/or PIs on different subbands have similarpolynomial functions, these subbands may be grouped into a subband set.In particular, if the variation of RPIs and/or PIs between subbands iswithin ER or EP, these subbands may be grouped into a subband set. ER orEP may be predetermined values; or DR or DP may be determined by asignaling from a base station. The terminal may report the values of N,Kn and Mn. A special case is when the K subbands are divided evenly intoN subband sets; in this case the terminal only needs to report the valueof N.

The method above can ensure a good feedback performance even whenchannel estimation is affected by frequency selective noise orinterference, and reduce feedback overhead.

Embodiment 3

This embodiment shows a particular example of channel informationfeedback. The base station sends a reference signal to the terminal. Theterminal reports CSI according to a measurement on the reference signal.CSIs of multiple subbands are to be reported on the CSI reporting band;the terminal selects, according to the result of channel measurement, aportion of the subbands for CSI feedback, and reports subband selectioninformation to the base station. The subband selection informationincludes at least one of:

1. Information indicating a selection of M subbands, where the Msubbands are selected for RPI and/or PI reporting from K subbands of aCSI reporting band;

2. Information indicating a division of N subband sets and a selectionof Mn subband sets, where the K subbands of a CSI reporting band aredivided into N subband sets, with the n-th subband set including Knsubbands, and Mn subbands are selected from the Kn subbands.

The terminal reports the information above on a PUCCH or PUSCH, in atleast one of the following ways:

1. In PUCCH feedback, the terminal reports M, N, Kn, and Mn relatedinformation in a first slot, and reports RPI and/or PI information in asecond slot, the first slot being prior to the second slot;

2. In long-PUCCH feedback, the terminal carries M, N, Kn, and Mn relatedinformation on a first OFDM symbol or symbol group, and carries RPIand/or PI information on a second OFDM symbol or symbol group, the firstOFDM symbol or symbol group being prior to the second OFDM symbol orsymbol group;

3. In PUSCH feedback, the terminal maps M, N, Kn, and Mn relatedinformation to a Resource Element (RE) near a front-loaded DMRS, andmaps RPI and/or PI information to a following RE.

4. M, N, Kn and Mn related information is mapped to a RE near the RImapping position;

5. M, N, Kn and Mn related information are jointly reported with RI.

According to an exemplary embodiment, the present disclosure alsoprovides a storage medium, more specifically, a computer-readablestorage medium such as a memory containing a computer program, thecomputer program being executable by a processor of a CSI feedbackapparatus to perform the steps in the methods above. Thecomputer-readable storage medium may include Ferromagnetic Random AccessMemory (or, Programmable Read-Only Memory, FRAM), Read Only Memory(ROM), Programmable Read-Only Memory (PROM), Erasable ProgrammableRead-Only Memory (EPROM), Electrically Erasable Programmable Read-OnlyMemory (EEPROM), Flash Memory, Magnetic Storage, Optical Disk, orCompact disc read-only memory (CD-ROM), including computer-executableinstructions, such as program code, executed by computer.

While the embodiments of the present disclosure contains many specifics,these should not be construed as limitations on the scope of anyinvention or of what may be claimed, but rather as descriptions offeatures that may be specific to particular embodiments of particularinventions. Various modifications and adaptations may be made by thoseskilled in the art without departing from the scope of the invention asdefined in the appended claims. The scope of the invention, therefore,shall be defined solely by the following claims.

What is claimed is:
 1. A method for wireless communication, comprising:performing, by a terminal device, a channel measurement based on aChannel State Information (CSI) reference signal from a base station;and configuring, by the terminal device, a CSI feedback report based onthe channel measurement as one of a wideband feedback or a subbandfeedback according to a configuration from the base station, wherein theCSI feedback report includes a phase coefficient indicator and anamplitude coefficient indicator that are associated with a precodingcodebook index indicating a precoding codebook that is a linearcombination of a set of vectors, and wherein, in response to the CSIfeedback report being configured as a subband feedback, the CSI feedbackreport includes information of M subbands selected from K subbands of aChannel State Information (CSI) reporting band, wherein M and K areintegers greater than or equal to 1, wherein M<K, and wherein the Msubbands are in a comb-shaped distribution in the K subbands; andtransmitting, by the terminal device, the CSI feedback report to thebase station.
 2. The method of claim 1, wherein the M subbands areselected from the K subbands by: dividing the K subbands evenly into oneor more subband sets; and selecting one subband from each of the one ormore subband sets.
 3. The method of claim 1, wherein the phasecoefficient indicator is associated with a weighted coefficient used inthe linear combination of the set of vectors.
 4. The method of claim 1,wherein the phase coefficient indicator and the amplitude coefficientare reported as a precoding matrix indicator (PMI) in the CSI feedbackreport.
 5. A method for wireless communication, comprising: receiving,by a base station, a Channel State Information (CSI) feedback reportfrom a terminal device based on a channel measurement, wherein the CSIfeedback report includes a phase coefficient indicator and an amplitudecoefficient indicator that are associated with a precoding codebookindex indicating a precoding codebook that is a linear combination of aset of vectors, wherein the CSI feedback report corresponds to awideband feedback or a subband feedback based on a configuration fromthe base station, and wherein, in response to the CSI feedback reportbeing configured as a subband feedback, the CSI feedback report includesinformation of M subbands selected from K subbands of a Channel StateInformation (CSI) reporting band, wherein M and K are integers greaterthan or equal to 1, wherein M<K, and wherein the M subbands are in acomb-shaped distribution in the K subbands.
 6. The method of claim 5,wherein the M subbands are selected from the K subbands by: dividing theK subbands evenly into one or more subband sets; and selecting onesubband from each of the one or more subband sets.
 7. The method ofclaim 5, wherein the phase coefficient indicator is associated with aweighted coefficient used in the linear combination of the set ofvectors.
 8. The method of claim 5, wherein phase coefficient indicatorand the amplitude coefficient are reported as a precoding matrixindicator (PMI) in the CSI feedback report.
 9. An apparatus for wirelesscommunication, comprising a processor that is configured to: perform achannel measurement based on a Channel State Information (CSI) referencesignal from a base station; and configure a CSI feedback report based onthe channel measurement as one of a wideband feedback or a subbandfeedback according to a configuration from the base station, wherein theCSI feedback report includes a phase coefficient indicator and anamplitude coefficient indicator that are associated with a precodingcodebook index indicating a precoding codebook that is a linearcombination of a set of vectors, and wherein, in response to the CSIfeedback report being configured as a subband feedback, the CSI feedbackreport includes information of M subbands selected from K subbands of aChannel State Information (CSI) reporting band, wherein M and K areintegers greater than or equal to 1, wherein M<K, and wherein the Msubbands are in a comb-shaped distribution in the K subbands; andtransmit the CSI feedback report to the base station.
 10. The apparatusof claim 1, wherein the M subbands are selected from the K subbands by:dividing the K subbands evenly into one or more subband sets; andselecting one subband from each of the one or more subband sets.
 11. Theapparatus of claim 9, wherein the phase coefficient indicator isassociated with a weighted coefficient used in the linear combination ofthe set of vectors.
 12. The apparatus of claim 1, wherein the phasecoefficient indicator and the amplitude coefficient are reported aprecoding matrix indicator (PMI) in the CSI feedback report.
 13. Anapparatus for wireless communication, comprising a processor that isconfigured to: receive a Channel State Information (CSI) feedback reportfrom a terminal device based on a channel measurement, wherein the CSIfeedback report includes a phase coefficient indicator and an amplitudecoefficient indicator that are associated with a precoding codebookindex indicating a precoding codebook that is a linear combination of aset of vectors, wherein the CSI feedback report corresponds to awideband feedback or a subband feedback, and wherein, in response to theCSI feedback report being configured as a subband feedback, the CSIfeedback report includes information of M subbands selected from Ksubbands of a Channel State Information (CSI) reporting band, wherein Mand K are integers greater than or equal to 1, wherein M<K, and whereinthe M subbands are in a comb-shaped distribution in the K subbands. 14.The apparatus of claim 13, wherein the M subbands are selected from theK subbands by: dividing the K subbands evenly into one or more subbandsets; and selecting one subband from each of the one or more subbandsets.
 15. The apparatus of claim 13, wherein the phase coefficientindicator is associated with a weighted coefficient used in the linearcombination of the set of vectors.
 16. The apparatus of claim 13,wherein phase coefficient indicator and the amplitude coefficient arereported as a precoding matrix indicator (PMI) in the CSI feedbackreport.